Superabsorbent polymers (SAPs) are synthetic polymer materials that are able to absorb about 500 to 1000 times their own weight in moisture. Such superabsorbent polymers have begun to be used in real-world applications for sanitary items, and are currently being widely utilized not only in hygiene products, such as disposable baby diapers, sanitary pads and the like, but also in soil conditioners for gardening applications, water stopping agents for civil engineering and construction applications, sheets for raising seedlings, freshness preservatives for food distribution, fomentation materials, etc.
In the preparation of the superabsorbent polymer, water is a polymerization medium, and is used in various applications, including facilitating the dispersion of the crosslinking solution during the surface crosslinking process, etc. Also, residual moisture in the final product functions as an anti-static agent and a plasticizer for resin, and plays a role in suppressing the generation of very small superabsorbent polymer dust in the course of application processing and additionally preventing the grinding of the superabsorbent polymer particles. Generally, however, when water is added even in a small amount to the superabsorbent polymer, the surface stickiness of the polymer may be increased by the water absorbed on the surface thereof, and irreversible agglomeration of the superabsorbent polymer particles may take place. This increase in stickiness and agglomeration may result in poor processability, imposing a burden on preparation and application processes, consequently increasing the particle size of the superabsorbent polymer, deteriorating the properties thereof, and decreasing productivity. Superabsorbent polymers have been studied to date in terms of the polymerization process thereof and enhancements in absorption capacity thereby, and surface crosslinking for increasing the surface properties of the superabsorbent polymer or absorption under pressure thereof. Furthermore, research into changes in the surface properties of the superabsorbent polymer has been carried out in order to increase permeability or prevent caking upon storage (anti-caking).
Particularly, superabsorbent polymers, having a crosslinked three-dimensional mesh structure, swell in water but are not dissolved in water. Hence, core/shell structure techniques for increasing the crosslinking density of the surface of the superabsorbent polymer are being developed to assure permeability and high absorption under pressure. Through such techniques, final properties are determined depending on a variety of conditions, including the crosslinking density, uniform shell formation, the thickness of the shell, etc. Since the superabsorbent polymer having a core-shell structure is weak in attrition resistance, the properties thereof, especially liquid permeability, may be deteriorated after diaper processing, and the generation of fine powder may be increased during diaper processing. Hence, minimizing such problems is very important in improving diaper performance.
In this regard, Japanese Patent Application Publication No. 2006-526691 discloses a superabsorbent polymer, the surface of which is modified. However, limitations are imposed on improving attrition resistance, permeability and absorption speed by locating water in the particles. Accordingly, there is a need to develop a method of preparing a superabsorbent polymer having superior properties.